Self triggered imaging device for imaging radiation

ABSTRACT

A semiconductor radiation imaging assembly comprises a semiconductor imaging device including at least one image element detector. The imaging device is arranged to receive a bias for forming the at least one image element detector. The assembly also includes bias monitoring means for monitoring the bias for determining radiation incident on the image element detector. Preferably, the imaging device comprises a plurality of image element detectors the bias for at least some of which is monitored for determining incident radiation. More preferably, the bias for all the detector elements is monitored.

This invention relates to a self-triggering imaging assembly for imagingradiation and to a self-triggerable imaging system.

BACKGROUND OF THE INVENTION

Imaging devices comprising an array of image elements of various typesare known

Charged coupled image sensors (also known as charged coupled devices(CCDs)) form one type of known imaging device. A CCD type deviceoperates in the following way:

1. Charge is accumulated within a depletion region created by an appliedvoltage. For each pixel (image cell) The depletion region has apotential well shape and constrains electrons under an electrode gate toremain within the semiconductor substrate.

2. Voltage is applied as a pulse to the electrode gates of the CCDdevice to clock each charge package to an adjacent pixel cell. Thecharge remains inside the semiconductor substrate and is clockedthrough, pixel by pixel, to a common output.

During this process, additional charge cannot be accumulated.

Another type of imaging device which is known is a semiconductor pixeldetector which comprises a semiconductor substrate with electrodes whichapply depletion voltage to each pixel position and define a chargecollection volume. Typically, simple buffer circuit read out theelectric signals when a photon is photo-absorbed or when ionisingradiation crosses the depletion zone of the substrate. Accordingly pixeldetectors of this type typically operate in a pulse mode, the numbers ofhits being accumulated externally to the imaging device. The buffercircuits can either be on the same substrate (EP-A-0,287,197) as thecharge collection volumes, or on a separate substrate (EP-A-0,571,135)that is mechanically bonded to a substrate having the charge collectionvolumes in accordance with, for example, the well known bump-bondingtechnique.

A further type of device is described in International applicationWO95/33332. In WO95/33332, an Active-pixel Semiconductor Imaging Device(ASID) is described. The ASID comprises an array of image elementsincluding a semiconductor substrate having an array of image elementdetectors and a further array of image element circuits. The imageelement detectors generate charge in response to instant radiation. Eachimage element circuit is associated with a respective image elementdetector and accumulates charge resulting from radiation incident on theimage element detector. The image element circuits are individuallyaddressable and comprise circuitry which enables charge to beaccumulated from a plurality of successive radiation hits on therespective image element detectors. The device operates by accumulatingcharge on the gate, for example, of a transistor. Accordingly, analoguestorage of the charge value is obtained. At a determined time, thecharge from the image element circuits can be read out and used togenerate an image based on the analogue charge values stored in each ofthe image element circuits.

CCD devices suffer from disadvantages of limited dynamic range, due tothe limited capacity of the potential well inside the semiconductorsubstrate, and also to the inactive times during which an image is readout. Pulse counting semiconductive pixel devices also have thedisadvantage of limited dynamic range. As these devices read the pixelcontact when a hit is detected, they suffer from saturation problems athigh counting rates. The semiconductor image element device according toWO95/33332 provides significant advantages over the earlier prior art byproviding a large dynamic range for the accumulation of images.

It has been proposed to utilise the above-mentioned CCD andsemiconductor devices to replace the film used in conventional radiationimaging systems, in order to provide real-time imaging and a morecontrolled lower dosage of radiation for a given exposure.

In a known arrangement, a CCD is electrically connected to an X-raysource. When the X-ray source is energised a start signal is transmittedalone the connecting wire to the CCD and its control circuitry to beginimage acquisition and read-out.

In a optional arrangement disclosed in U.S. Pat. No. 5,513,252 there isno connection to the X-ray source. Instead, the CCD is continuallyread-out prior to radiation. A signal derived from the CCD is comparedwith a reference level. If the signal exceeds the reference level, theimage acquisition of the CCD is initiated, that is to say the CCD stopsbeing read out and the image starts to accumulate on the CCD.

European Patent Application Publication No. 0 756 416 A1 discloses a CCDused as an imaging device in which charge accumtulated in the CCDelements is clocked from several rows into a register in order to sumthe charges. The summed result is put to a threshold test. Onset ofX-ray radiation is detected when the signal applied to the thresholdtest exceeds a reference level. Image acquisition is then initiated, asdescribed above i.e. only then will the CCD start accumulating theimage.

In yet another arrangement the X-ray source and CCD have again nophysical connection. A further sensor is arranged close to the imagingarray for the CCD to detect the onset of X-ray radiation. On detectionof incident X-ray energy, the sensor sends a signal to the CCD controlcircuitry to initiate image acquisition, as before.

The foregoing prior art systems involve a delay between activation ofthe radiation source and initiation of image acquisition. Since inradiation imaging, in particular X-ray imaging, the exposure toirradiation and radiation devices should be kept as low as possible itis desirable to reduce the delay as much as possible. Furthermore, theCCD approach is unsuitable for determining an end of an exposure. Anadditional sensor or a connection to the radiation source is necessaryto provide an exposure trigger indicating end of irradiation.

SUMMARY OF THE INVENTION

In accordance with an embodiment according to a first aspect of theinvention there is provided a semiconductor radiation imaging assembly,comprising: a semiconductor imaging device including at least one imageelement detector, said imaging device arranged to receive a bias forforming said image element detector; and bias monitoring means formonitoring said bias for determining radiation incident on said imageelement detector.

In accordance with an embodiment according to a second aspect of theinvention, there is provided a method for providing a semiconductorimaging assembly, including an image element detector, comprising:monitoring a bias for said image element detector to determine radiationincident on said image element detector; and initiating a trigger forsaid bias fulfilling a predetermined condition.

In accordance with an embodiment according to a third aspect of theinvention, there is provided a self-triggerable semiconductor radiationimaging system, comprising: a semiconductor imaging assembly as oroperable as described in the foregoing paragraphs; control electronicscoupled to said imaging assembly for receiving signals, includingtrigger signals, therefrom; signal storage means for storing signalscoupled from said control electronics; an image processor for processingsignals coupled from said control electronics; and a display unit fordisplaying images provided by said image processor.

Embodiments in accordance with the first, second or third aspects of theinvention advantageously provide a substantially instantaneous orreal-time response to radiation incident on an or a plurality of imageelement detectors by monitoring the bias applied to form the imageelement detector/s. Such embodiments may provide trigger signals indirect response to radiation incident on the image element detector/s,yet by indirect monitoring of the incident radiation, thereby obviatingthe need for reading out data from the image elements. Further, theembodiments provide for self-triggering image detector devices andsystems, and obviate the need for trigger signals to be provided fromX-ray sources to the control electronics of such systems for identifyingbeginning and/or ending of exposure.

Furthermore, since the bias represents an average of the radiationincident over the whole area of an array of image elements it provides arobust indication of the total radiation incident over that area, andprovides a sensitive self-triggering mechanism.

Suitably, the semiconductor imaging device comprises a semiconductorsubstrate supporting a first and second conductive layer on respectivefirst and second surfaces. The first and second conductive layers atleast partially oppose each other for applying the bias between them toform a radiation detection zone for the image element detector.

Typically, the first conductive layer comprises a substantiallycontinuous layer across the first substrate surface, and the secondconductive layer comprises a plurality of image element electrodes fordefining respective radiation detection zones for a plurality of imageelement detectors.

Advantageously, the bias monitoring means is adapted to provide atrigger for the bias fulfilling a predetermined criterion.

In accordance with a first preferred embodiment, the bias monitoringmeans determines a rate or direction of change of the bias, and morepreferably discriminates between different rates or direction of change.Thus, increases and decreases in bias due to corresponding increases anddecreases in incident radiation intensity may be determined and utilisedto initiate suitable trigger signals.

Preferably, one or more threshold values are set corresponding to biaslevels representative of incident radiation levels at start of exposureand/or end of exposure of which triggers are to be initiated. Suchtriggers are initiated for the bias transgressing respective bias levelsat and/or in an appropriate direction.

The bias monitoring means preferably monitors the bias current, andprovides a signal representative of the bias current, although for anassembly having the bias supplied from a non-constant output voltagesupply the bias voltage may be monitored. This representative signal isdifferentiated, the resulting signal low-pass filtered (by an integratorfor example) and the filter result input to a comparator for comparingwith one or more threshold levels. Thus, typically the threshold levelsare compared against a value of an intermediate signal representing theincident radiation and derived from the bias current.

An example of a suitable differentiator is a high pass filter, and anexample of a suitable low pass filter is an integrator. Cut-offfrequencies for the high pass filter differentiator and the low passfilter integrator suitably lie in the ranges 10-200 Hz. and 500 Hz.-2kHz. respectively.

In accordance with a second preferred embodiment of the invention, thebias monitoring means accumulates a bias value which represent aggregateradiation incident on the image element detector/s, which advantageouslyoffers improved radiation detection rejection against reliability. Thus,the likelihood of false positive triggers is reduced without adverselyaffecting the sensitivity to incident radiation.

Preferably, an image element dark or quiescent bias value is subtractedfrom a value representing the bias, and the resulting signal is thenintegrated to provide an indication of the total accumulated bias valueafter correction for the quiescent bias value. One or more thresholdlevels arc set against which the accumulated bias value is compared, toinitiate suitable trigger signals such as start of and/or end ofexposure trigger signals. Preferably, for an imaging device comprisingmore than one image element detector, then a dark or quiescent biasvalue corresponding to all of the individual image elements issubtracted from the value representing the bias value.

Suitably, sample and hold circuitry is configured to record the biasvalue prior to a radiation exposure in order to obtain a suitablequiescent bias value.

In accordance with a third preferred embodiment of the invention, anintegrated signal representative of the bias, preferably bias currentbut optionally voltage, is subtracted from the signal representative ofthe bias to derive a signal corresponding to the radiation incident onthe image element detector/s. The resulting signal is then compared withthreshold values to initiate suitable trigger signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which like elementshave like reference signs and in which:

FIG. 1 is a schematic block diagram of an overall imaging configuration;

FIG. 2 is a cross-section of one example of an imaging device;

FIG. 3 is a schematic diagram of a plan view of corner of the imagedevice of FIG. 2;

FIG. 4 is a schematic diagram of an image element circuit in accordancewith an embodiment of the invention;

FIG. 5 is a schematic diagram of another example of an image elementcircuit for a further embodiment of the invention;

FIG. 6 is a block diagram of differential (edge) detection circuitry inaccordance with a first embodiment of the invention;

FIG. 7 illustrates the signals obtained at various points on thecircuitry illustrated in FIG. 6;

FIG. 8 is a block diagram of a dose detection trigger circuitry inaccordance with a second embodiment of the invention;

FIG. 9 illustrates signals obtained at various points in the circuitryillustrated in FIG. 8;

FIG. 10 is a block diagram of further dose detection trigger circuitryin accordance with a third embodiment of the invention;

FIG. 11 illustrates signals obtained at various points in the circuitryillustrated in FIG. 10;

FIG. 12 is a schematic circuit diagram of a bias current measurementsuitable for embodiments of the present invention;

FIG. 13 is a schematic circuit diagram of a second order high passfilter suitable for a differentiator in accordance with embodiments ofthe invention;

FIG. 14 is a schematic circuit diagram of a comparator suitable forembodiments of the present invention; and

FIGS. 15-24 illustrate oscilloscope plots of an integrated signal forvarious configurations of circuitry substantially in accordance with thefirst embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic block diagram of an example of an imaging systemsuitable for use with the present invention. This particular embodimentis directed to the imaging of high energy radiation, for example X-rayradiation. By high energy radiation is meant radiation having an energyin excess of approximately 1 KeV. However, the invention is by no meanslimited to high energy radiation such as X-rays but could be applied tothe detection of any particular radiation, for example γ-ray, β-ray,α-ray, infra-red or optical radiation, subject to an appropriate choiceof semiconductor substrate.

The imaging system 10 of FIG. 1 is shown to provide imaging of an object12 subject to radiation 14. In this example the radiation may, forexample, be X-ray radiation as mentioned above, but could alternativelybe γ-ray, β-ray or α-ray radiation. The object 12 may, for example, bepart of a human body. The imaging device 16 comprises a plurality ofimage elements (here elements 18 of a two dimensional image elementarray). In the following, reference will be made to image elements 18,although it will be appreciated that in other embodiments the individualimage elements may have a configuration other than that of an elementwithin a two dimensional array (eg a strip arrangement).

Control electronics 24 includes processing and control circuit forcontrolling the operation of the imaging device, or an array of imagingdevices. The control electronics 24 is interfaced to the imaging device16 via path 22 and enables the readout circuits 20 associated withindividual image elements 18 to be addressed (e.g. scanned) for readingout charge from the readout circuits 20 at the individual image elements18. The charge readout is supplied to Analog-to-Digital Converters(ADCS) for dicitisation and Data Reduction Processors (DRPs) forprocessing the digital signal. A bias 110 for the imaging device mayalso be applied over interface 22. Optionally, the bias may be appliedvia some other path.

The control electronics 24 is further interfaced via a path representedschematically by the arrow 26 to an image processor 28. The imageprocessor 28 includes data storage in which it stores digital valuesrepresentative of the charge values read from each image element alongwith the position of the image element 18 concerned. The image processor28 builds tip an image for display The data storage can store digitalvalues for up to two image frames, i.e. signals corresponding to twoimages. Later signals overwrite or force out in a first in first outmanner previously stored signals. A trigger signal determines where inthe data storage the first signal for an exposure is located, and startsthe read out of data from this location and then reads out the next n−1data storage locations corresponding to the remaining n−1 image elementsof an n element imaging system. For a start of exposure trigger, thefirst read out storage location corresponds to the start of exposuretrigger. For an end of exposure trigger the image processor works backfrom the storage location corresponding to the trigger signal, to readout the remaining image frame signals corresponding to the radiationexposure. The values are read out by the image processor 28 to cause arepresentation of the data to be displayed on a display 55 via a pathrepresented schematically by the arrow 56. The data can, of course, beprinted rather than, or in addition to, being displayed and can besubjected to further processing operations, including non-volatilestorage in magnetic media for example. Input devices 58, for example, akeyboard and/or other typical computer input devices, are provided forcontrolling the image processor 28 and the display 55 as represented bythe arrows 59 and 61.

The imaging device detects directly high energy incident radiation andaccumulates at each image element, a count of the incident radiationhits at that image element.

The imaging device can be configured as a single semiconductor substrate(eg, of silicon) with each image element comprising an image elementdetector 19 and image element circuitry 20. Alternatively, the imagingdevice 16 can be configured on two substrates, one with an array ofimage element detectors and one with an array of corresponding imageelement circuits 20, the substrates being mechanically connected to eachother by, for example, conventional bump-bonding technology or any otherappropriate technology.

FIG. 2 is a schematic cross section of part of an imaging device 16. Inthis example, the imaging device 16 comprises an image detectorsubstrate 30 connected to an image circuit substrate 32 by means ofbump-bonds 34. An image element detector 19 of each image clement 18 isdefined on the detector substrate 30 by a continuous electrode 36 whichapplies a biasing voltage and image element location electrodes 38,which collect charge, to define a detection zone for the image element18. Such a detection zone comprises a charge generation volume in whichcharge is generated responsive to incident radiation, and caused todrift to respective electrodes under the influence of the bias.Preferably, a passivation material 21 such as aluminium nitride, siliconnitride or silicon oxide for example, is disposed between adjacent imageelement electrodes 38. Corresponding image element circuits 20 on theimage circuit substrate 32 are defined at locations corresponding to theelectrodes 38 (ie to the image element detectors 19). The image elementcircuits 20 are electrically connected to the corresponding electrodes38 by bump-bonds 34. In this manner, when charge is generated in animage element detector 19 in response to incident radiation, this chargeis passed via the bump-bond 34 to the corresponding image elementcircuit 20.

A continuous electrode 36 may be fabricated from suitably conductivematerial such as aluminium, gold, indium/platinum alloy or platinum/goldalloy, for example. The image element electrodes may be fabricated froma conductive material such as gold, platinum/gold alloy or nickel/goldalloy, for example.

Each image element 18 of the imaging device 16 is in effect defined onthe substrate by electrodes 38 which apply a biasing voltage incooperation with continuous electrodes 36 to define a detection zone(i.e. the image element detector 19) for the image element 18.Corresponding readout circuits on the readout substrate can comprise,for example, active image element circuits 20 as described in theaforementioned WO 95/33332. The image element detectors 19 are formedwith a detection zone such that, when a photon is photo-absorbed in thesemiconductor substrate 30 at an image element 18 creating an electriccharge or when a charged radiation ionises the detection zone of thesemiconductor substrate 30 at an image element 18, an electric pulseflows from the semiconductor detection zone to the readout circuit 20for that image element 18 through the solder bump 34 for that imageclement.

The actual size of the image element circuit and the image elementdetector will depend on the application for which the imaging device isintended.

As mentioned above, the image element detectors and image elementcircuits could be constructed integrally on a single semiconductorsubstrate. Such an implemrntation is possible, but sets challengesunrelated to the present invention, relating to circuit manufacturingtechniques. With suitable circuit manufacturing techniques, theinvention as described herein is perfectly applicable to implementationon a single semiconductor substrate, as opposed to the dual-substratetechnique described herein.

Any appropriate semiconductor materials can be used for the substrates.For example, silicon may be used for the detector substrate and for theimage circuit substrate. Other semiconductor materials could be used.For example, for the detector substrate, the material could be selectedfrom: CdZnTe, CdTe, HgI₂, InSb, GaAs, Ge, TIBr, Si and PbI.

By way of example, for a detector substrate material of CdZnTe thecontinuous electrode 36 is typically held at a voltage in the range of−100V to −600V relative to a reference voltage, whilst for CdTe, thevoltage is +/−100V to +/−600V. An electric field of approximately±200V/mm is applied between the continuous electrode 36 and imageelement electrodes 38 which are held at a voltage of about −/+5V or +3V.For a silicon detector substrate an electric field of +150V/0.5 mm isapplied between the continuous electrode 36 and image element electrodes38.

When an X-ray photon is photo-absorbed in a detection zone of imageelement detector 19 an electric charge is created, (or charged particleor gamma-ray is incident or absorbed for other embodiments) an electricpulse flows from image element detector 19 via the bum-bonds 34 tocorresponding image element circuitry 20.

FIG. 2 also shows an optional guard ring area 48 integral with thedetector substrate 30.

The guard ring area 48 generally surrounds all the image elements 18,and may comprise several guard rings 50. The guard rings 50 are madefrom a conductive material, preferably the same material as used forimage element location electrodes 38 in order for the guard rings to befabricated at the same time as the image element electrodes 38. Theguard ring area 48, defined by guard rings 50, reduces charge injectiondue to crystal defects at the edge of the detector substrate 30 byreducing the localised increase in field strength at the edge for thedetector substrate 30 material. The guard ring also helps to maintain auniform electric field inside the detector substrate 30.

Continuous electrode 36 extends into the guard ring area 48 and alsoapplies a bias voltage in the guard ring area 48. This forms a furtherdetection zone hereinafter referred to as radiation detection cell 57,between the guard ring 48/50 and the continuous electrode 36. The guardring 48/50 is exposed to radiation when the imaging device isirradiated, as well as the image element detectors 19.

FIG. 3 shows a corner 60 of an example of a detector substrate 30including a guard ring. Image element location electrodes 38 aredisposed inside the guard ring area 48. As mentioned earlier, radiationincident on the imaging device 16 falls not only on image elementlocation electrodes 38, but also on guard rings 50 comprising the guardring area 48. The guard ring area 48 may comprise more than one guardring 50 for gradually reducing the field strength towards the edge ofthe detector substrate 30. The guard ring 50 is coupled to outputcircuitry 54 via a bump-bond 52, which is directly coupled to aradiation sensor output pin on the imaging device.

FIG. 4 illustrates one preferred example of an image element circuit 20for an image element in an embodiment of an imaging device in accordancewith the invention. This example uses field effect transistors (FETs)arranged as a cascode connected amplifier. VBLAS 90 is a bias voltageinput across the depletion zone forming the image element detector 19 ofthe image element. The image element detector 19 is represented by thediode symbol D11. In the image element itself, SIGOUT 92 is an analoguesignal output and VANA 94 is an analogue power supply input. RES-R-1 isa reset input and ENA-R-1 is an enable input for the image elementcircuit. Charge is accumulated in the gate of a transistor M11A 100 whenboth the RES-R-1 96 and ENA-R-1 98 inputs are low.

It will be appreciated that the use of a FET provides an example only ofan embodiment of the invention in which charge accumulating capacitanceis maximised using an image element charge storage device (such as a FETgate or a capacitor) that accounts for most of the input nodecapacitance for each pixel.

To read the pixel cell, ENA-R-1 is taken to a high state, which allowscurrent to flow from the transistor M11A 100 through the transistor M11B102 to SIGOUT 92. The pixel circuit is reset by taking RES-R-1 to high,whereupon after RES-R-1 has been at high for merely a few microseconds,any accumulated charge will have been removed from the gate of thetransistor M11A 100. Immediately after RES-R-1 96 goes to a low level,charge can begin to accumulate at the gate of the transistor M11A 100.If no reset pulse is supplied to the reset input RES-R-1 96, then it isto be noted that a reading operation when the enable input ENA-R-1 goeshigh does not destroy the charge but instead merely causes a currentflow directly proportional to the accumulated charge. This allowsmultiple readings without resetting.

FIG. 5 illustrates a further example of an active image element circuit320 for an image element in an embodiment of an imaging device inaccordance with the invention. This example is similar to the example ofFIG. 4. The image element detector is represented at PD 319 of the imageelement. In the image element circuit itself, VBLAS 340 is a voltagebias, OUT 342 is an analogue signal output, RESET 346 is a reset inputconnected to a reset FET 347 and ENABLE 348 is an enable input connectedto an enable FET 352 for the image element circuit. Charge (electrons)is (are) accumulated in the gate of a charge storage FET 350 when theENABLE 348 input is low and the RESET 346 input is high. To read theimage element, ENABLE 348 is taken to a high state, which allows currentto flow from the FET 350 through the FET 352 to OUT 342. The imageelement circuit is reset by taking RESET to low, whereupon after RESET346 has been at low for merely a few microseconds, any accumulatedcharges will have been removed from the gate of the FET 350. Immediatelyafter RESET 346 goes to a high level, charge can begin to accumulate atthe gate of the PET 350. If no reset pulse is supplied to the resetinput RESET 346, then it is to be noted that a reading operation whenthe enable input ENABLE goes high does not destroy the charge butinstead merely causes a currently flow directly proportional to theaccumulated charge. It will therefore be seen that the operation of thecircuit of FIG. 5 is similar to that of FIG. 4. In addition, the circuitof FIG. 5 includes diodes 354 and 356 which act as overload protectioncircuitry for the image element circuit. The diodes provide protectionboth against static electricity which might damage the FETs and againstFET overload. If the FET gate 350 accumulates more than a predeterminedcharge threshold (e.g. corresponding to 5 volts, which is the voltagebias), then current will start to flow through the diode 356 towards theground, thus protecting the FET 350. This will protect image elementswhich, for example, receive a full radiation does outside the perimeterof an object to be imaged. Preferably, the two FETs 350 and 352 areimplemented as a cascade amplifier stage. In this configuration, the twoFETs 350 and 352 provide impedance-up conversion without increasing thenoise accordingly. Consequently, the noise level from each pixel circuitdescribed in the current embodiment is only about 500 e while the pixelcircuit retains very small size (as small as 10-20 μm pixel size), verylarge dynamic range of about 50,000,000 e and individual addressability.

FIG. 5 also illustrates an optional bipolar transistor 360, which may beomitted.

In accordance with exemplary embodiments of the invention, bias 110 issupplied via the interface 22 with control electronics 24 to imagingdevice 16 to form bias voltage VBIAS 90 and VBIAS 340 described withreference to respective examples of the image elements 18 illustrated inFIGS. 4 and 5. Optionally, the bias 110 may be supplied directly from apower supply or other suitable module. The exemplary embodiments will bedescribed with reference to the monitoring of bias current. However, fora power supply having a non-constant voltage, voltage may be monitored.

Referring now to FIG. 6, there is schematically illustrated circuitry inaccordance with a first embodiment of the invention for providing atrigger signal for the imaging system 10, in particular the controlelectronics 24 described with reference to FIG. 1. The circuitryillustrated in FIG. 6 is differential or edge detection circuitry. Thebias current applied by bias 110 to the continuous electrode 36 ismeasured by bias current measurement unit 120. A bias current signal 122corresponding to the measured bias current is output from bias currentmeasurement unit 120 and input to a differentiator (high pass filter)124. The high pass filter 124 acts to differentiate the bias currentsignal 122. In the illustrated embodiment, high pass filter 124 has acut-off frequency of 100 Hz, but it will be readily apparent to theordinarily skilled person that the cut-off frequency need not be 100 Hzbut any suitable frequency capable of providing a suitabledifferentiation of expected or anticipated forms of bias current signal122. For example, a suitable high pass cut-off frequency may be in therange 10-200 Hz.

A differentiated bias current signal 126 is output from high pass filter124, and coupled to low pass filter (integrator) 128. In the illustratedembodiment, low pass filter 124 has a cut-off frequency of 1 kHz but thecut-off frequency may be any suitable frequency providing a suitable lowpass filtering function for expected or anticipated ranges ofdifferential bias current signals 126. For example, a suitable low passcut-off frequency may be in the range 500 Hz-2 kHz.

Low pass filter (integrator 128) couples an integrated signal 130 tocomparator 132 which compares integrated signal 130 with one or morethreshold values. Responsive to integrated signal 130 transgressing athreshold value, comparator 132 outputs a trigger signal 134 whichinitiates generation of a trigger pulse 136 for the imaging system.

An example of the variation over time of the X-ray flux energy orintensity of a typical X-ray source for medical/dental imaging during animaging or exposure sequence is illustrated in FIG. 7(a). The X-rayintensity illustrated in FIG. 7(a) comprises an initial pulse 140 whichis typically due to an impulse response or spikes from the X-ray powersupply and/or pre-heating of the X-ray tube filament when the X-raysource is activated or turn on to initiated an exposure.

Initial pulse 140 may be a predetermined time period 142 (e.g. 1 second)in advance of an X-ray exposure 144, in which case it may be utilised toinitiate a start of exposure trigger the predetermined time interval 142later. However, such a trigger would not be reliable since it may varywith age, use or temperature, for example, of the X-ray source.

The X-ray exposure profile 144 starts with a slowly increasing intensityhaving a slope 146, and which typically lasts for about 5-50milliseconds as illustrated. The slow increase in X-ray intensity istypically due to the gradual heating of the X-ray tubes and/or slowbuild-up of the X-ray source power supply voltage. Optionally, oradditionally, it may be due to positive control of the X-ray sourcepower supply to increase the lifetime of the X-ray source (tubefilament). The X-ray intensity flattens out to plateau 148, and finallyrapidly decreases with a large slope 150 at the end of the exposureprofile 144, for example over about 5 milliseconds as illustrated.

Generally, the causes of slow rise time 146 do not occur at the end ofthe exposure, and thus the X-ray intensity falls off rapidly.

FIG. 7(b) schematically illustrates the bias signal 122 as measured bybias current measurement unit 120 for the X-ray intensity profileillustrated in FIG. 7(a). Bias current signal 122 has a minimum level152 corresponding to an image device “dark” or “quiescent” current. Sucha “dark” or “quiescent” current is the current flowing through the imagedetector elements 19 due to the bias voltage applied thereto inconditions of no- or non-exposure levels of X-ray illumination. Such“dark” or “quiescent” currents provide an offset in the measured biascurrent signal profile 122. The bias current may vary due to temperaturevariations, although such variation is typically slow.

The bias current signal profile 122 comprises an initial pulse 154 andenvelope 156 having rising edge 158, plateau 160 and falling edge 162,corresponding to the X-ray intensity profile illustrated in FIG. 7(a).Changes in the bias current occur with changes in X-ray intensityincident on the imaging device 16, and are due to charge pairs generatedin the detection zone 19 of the detector substrate in response toincident radiation migrating to respective electrodes 36,38. The greaterthe intensity of X-rays the greater the number of charged pairs createdand thus the greater the measured bias current, and vice versa.

The signal 126 resulting from differentiating bias current signal 122 isillustrated in FIG. 7(c). Positive and negative square pulses 164 and166 respectively correspond to the rising (positive) gradient andfalling (negative) gradient edges of initial bias current signal pulse154. Positive square pulse 168 corresponds to rising edge 158 to thebias current signal profile 122, and negative square pulse 120corresponds to the falling edge 162. The effect of low pass filteringsignal 126 is illustrated in FIG. 7(d).

Although there are pulses (172, 174, 176, 178) in signal 130corresponding to the pulses (164, 166, 168, 170) in signal 126, the aremodified such that the pulse 174 corresponding to pulse 166 of signal126 is a lower amplitude than pulse 178 corresponding to pulse 170 ofsignal 126. By appropriate adjustment of the low pass filter (reducingthe cut-off frequency) pulse 172 may be attenuated (shown in brokenlines) with respect to pulse 176). Such modification would also changethe shape of pulses 176 and 178 (broken line) but they would stillachieve at least substantially the same amplitude as for an unmodifiedlow pass filter 128.

For a modified low pass filter signal 130 transgresses detection limit(1) 180, which corresponds to a first threshold value for comparator132, by pulse 176 only. The crossing of threshold 180 by respectiverising and falling edges of pulse 176 corresponds to points 182 and 184respectively on the X-ray exposure profile 144, and to correspondingthreshold values for the bias current signal illustrated in FIG. 7(b).Thus, respective transgressions of value 180 may be utilised to initiatea start of exposure trigger for use by the imaging system 10, dependingupon whether the start of exposure is determined to be just as the X-rayexposure begins, point 182, or where the X-ray intensity has reached asubstantially constant level, point 184. Comparator 132 may comprisediscrete logic circuitry hardwired to respond to one or othertransgressions of threshold 180 to initiate trigger pulse unit 136 toproduce a start of exposure trigger signal for control electronics 24 ofthe imaging system.

As is also illustrated in FIG. 7(d) negative going pulse 178transgresses a second threshold level 186 (detection limit (2)). Pulse178 crosses threshold 186 on both falling and rising edges,corresponding to point 188 and 190, respectively in the X-ray intensityprofile illustrated in FIG. 7(a). Typically, comparator 132 isconfigured to respond to transgressions of threshold 186 by the fallingedge (first transgression) of pulse 178 to initiate trigger pulse unit136 lo produce an end of exposure trigger signal for the imaging system,indicating that the X-ray exposure is completed, and image readoutshould begin.

Comparator 132 need not be comprised of discrete logic circuitry but maycomprise a suitably configured programmable logic array or a suitablyprogrammed processing unit. Indeed, all the modules post bias currentmeasurement illustrated in FIG. 6 may be implemented in a suitablyprogrammed processing unit such as a field programmable gate array, oreven a general purpose processor. However, for operation, e.g data orclock frequencies greater than 100 kHz, an optimised ApplicationSpecific Integrated Circuit would be required.

Optionally, the low pass filter is unmodified and signal 130 isrepresented by the unbroken lines of FIG. 7(d). In such an embodiment,either comparator 132 is configured to discard transgressions ofthreshold 180 by pulse 172, or optionally, comparator 132 is configuredto respond to transgressions of threshold 186 only, thereby initiatingonly an end of exposure trigger signal. This may be achieved by settingonly a single threshold level 186 (detection limit (2)). Any desiredstart of exposure trigger for the imaging system would then need to beprovided by some other means.

Circuitry in accordance with a second embodiment of the invention isillustrated in FIG. 8. The second embodiment may be termed a dosedetection trigger circuit, since it provides a trigger pulse dependingon the accumulated radiation or radiation dose received by the imagingdevice 16.

Bias current measurement unit 120 and bias current signal 122 operate asdescribed with reference to the first embodiment illustrated in FIG. 6.Similarly, the X-ray intensity profile and bias current signal profile122 respectively illustrated in FIGS. 9(a) and 9(b) are as describedwith reference to FIGS. 7(a) and 7(b) above.

Bias current signal 122 is input to resettable sample and hold unit 192,and to subtraction circuitry 194. The output 152 of sample and hold unit192 is input to subtraction circuitry 194, and is subtracted from biascurrent signal 122. The sample and hold unit 192 also receives aninitial trigger signal 198 which resets unit 192 and may be utilised toreset unit 192 just prior to an X-ray exposure in order to store the“dark” or “quiescent” current value 152 just prior to the X-rayexposure. The initial trigger signal may correspond to an X-ray triggeroriginating from the X-ray source to indicate the start of an exposureor may occur periodically outside of an X-ray exposure window.

A dark current corrected signal 196 is input to a resettable integrator200, which is reset on receipt of initial trigger signal 198, andprovides an integrated signal 202 to comparator 204. Comparator 204outputs trigger signal 136 for the integrated signal 202 transgressing athreshold value for the comparator. The trigger signal 134 is coupled totrigger pulse unit 136 which provides a trigger pulse to the controlelectronics 20 for the imaging system 10.

As mentioned above, FIGS. 9(a) and 9(b) illustrate a typical X-rayintensity profile and bias current signal 122 as described withreference to FIGS. 7(a) and 7(b). FIG. 9(c) illustrates a sample, 152′,of the dark current 152 stored in response to an initial trigger 198shown in broken line on FIG. 9. The initial trigger 198 also resetsintegrator 200 as illustrated by reference to 206 in FIG. 9(d). Thesignal 202 output from integrator 200 has a profile as illustrated.Initially, the profile ramps up, 208, corresponding to initial X-raypulse 140, and flattens-out until exposure pulse 144 during which signal202 increases until the X-ray intensity drops off, 150. Comparator 204includes a threshold value 210 corresponding to the detection limitillustrated in FIG. 9(d). For signal 202 transgression threshold 210 atrigger signal 134 is output from comparator 204 to the trigger pulseunit 136. The detection limit is set at a value at least greater thanthe signal 202 value due to pulse 140, but thereafter may be set at anysuitable level to initiate a trigger pulse imaging system 10. A suitabledetection limit 210 would result in signal 202 transgressing a thresholdat point 212, which corresponds to the start of constant X-ray intensityillumination, labelled 214 in FIG. 9(a), and to a correspondingthreshold for the bias current signal illustrated in FIG. 9(b). Such atrigger would be a start of exposure trigger. An end of exposure triggermay be automatically generated by the imaging system, a predeterminedtime period after the start of exposure trigger signal. Optionally,plateau 216 may be identified by the system in order to initiate an endof exposure trigger, or a second threshold 215 may be set for thecomparator 204, the transgression of which initiates an end of exposuretrigger pulse.

A third embodiment of the invention comprises a second dose detectiontrigger circuit as illustrated in FIG. 10. A second dose detectiontrigger circuit replaces the sample and hold unit 192 and integrator 200of the circuitry illustrated in FIG. 8, with simple low pass filters 220and 228, respectively. Utilising such a configuration obviates the needfor an initial trigger and thereby reduces the complexity of the imagingsystem 10 control circuitry 24.

As illustrated in FIG. 10, bias current signal 122 is input to low passfilter 220, preferably having a cut-off frequency in the range 10-200 Hzfor example, and to subtraction circuitry 224. Subtraction circuitry 224subtracts the low pass filtered bias current signal 222 from the rawbias current signal 122 to produce signal 226 which is corrected for thedark current. This is due to the fact that since the dark current slowlyvaries over time (e.g. due to changes in temperature), the measured biascurrent profile after low pass filtering is substantially similar to itsunfiltered profile. The profile of signal 226 is illustrated in FIG.9(c).

Signal 226 is input to the second low pass filter 228, preferably havinga cut-off frequency in the range 10-200 Hz for example, resulting in asignal 230 having a profile as illustrated in FIG. 9(d). Comparator 232receives signal 230 from low pass filter 228, and compares it with athreshold value 234 (detection limit). As illustrated in FIG. 9(d),signal 230 upwardly transgresses threshold 234 at point 236corresponding to point 238 on the X-ray intensity profile 144. Thus, thecomparator can output a trigger signal to trigger pulse unit 136 toinitiate a start of exposure trigger.

Comparator 232 may also output a trigger signal corresponding to adownward transgression of threshold 234 at point 240 corresponding topoint 242 on the X-ray intensity profile 144, to initiate an exposuretrigger signal.

By passing signal 226 through a second low pass filter 228, themagnitude of signal corresponding to the initial pulse 140 is greatlyreduced, relative to the signal corresponding to the X-ray exposureprofile 144. Therefore, the likelihood of false triggering due to theinitial pulse 140 is reduced.

An example of bias current measurement circuitry suitable forembodiments of the invention, in particular edge detection circuitry,will now be described with reference to FIG. 12. The high voltage powersupply and electronic circuit of the imaging system are coupled to acommon reference potential, for example ground. Thus, bias measurementis undertaken by coupling a resistor 248 across the high voltage node250 for the image device 16 and high voltage node 252 from the powersupply. Optionally, it is possible to measure bias current directly fromground if the power supply is floating. Such optional measurementconfiguration advantageously allows DC coupling and implementation of acompensation measurement method.

The power supply node end of resistor 248 is coupled to the referencepotential (ground) via capacitor 254. The image device node end ofresistor 248 is coupled to the positive input 262 of operationalamplifier (op-amp) 260 via a capacitor 256 having a value C. A resistorhaving a value R, 258, is coupled between the positive input 262 and thereference potential. The time constant of the RC combination ofcapacitor 256 and resistor 258 is selected to be low relative to thetime constant of the subsequent stages of the circuitry, for example tothe time constant providing the cut-off frequency of the high-passfilter stage of the edge detection circuitry. The value of capacitor 254is much greater than the value of the capacitor 256. The value of thecurrent sense resistor 248 is selected to provide sufficient voltagebetween nodes 250 and 252 to provide a reliable signal for measurement,yet without unacceptably affecting the operation of the image devicedetectors, and being sufficiently less than the value R of resistor 258not to influence the time constant of the bias current measurementcircuitry.

The output of op amp 260 is coupled to a resistor chain comprisingresistors 266 and 268, the centre of which is fed back to the negativeinput 264 of the op amp 260 to provide a suitable feedback signal. Theoutput is then coupled to the next stage of the circuitry.

FIG. 13 illustrates a second order high pass filter suitable forproviding a differentiator for the edge detection circuitry describedabove. Although the present description refers to a second order filter,a first order filter or higher order filters may be used. Input 270receives bias current signal 122 from the bias current measurement unit120, and is fed into a C-R-C network 272. The output of the C-R-Cnetwork 272 is input to the positive input of the operational amplifier(op-amp) 274 which is also coupled to reference potential (ground) via aresistor 276. The output of the op-amp 274 is coupled to the referencepotential (ground) via a resistor network 278,280, centre tapped toprovide a feedback to the negative input of the op amp 274. The outputof the op-amp is also fed back to the network 272.

For a low pass filter, the capacitors are interchanged with theresistors.

The high and low pass filters are preferably implemented havingcritically damped characteristics. However, by changing the op amp gain,more complex filter characteristics may be achieved. For example, forcritical damping the gains equal 1.0. For a Bessel function the gainequals 1.268 and for a 3 dB Tscbebyscheff filter, the gain equals 2.234,for example.

An example of a comparator circuit suitable for embodiments of theinvention is illustrated in FIG. 14. An input signal is coupled via ACcoupling capacitor 282 to the negative input of operationalamplifier(op-amp) 284. The negative input is also coupled to referencepotential (ground) by resistor 286. AC coupling is advantageous toreject any DC offset voltage derived from the preceding operationalamplifier stage circuitry due to the operational amplifiercharacteristics. Since the particular implementation details of thecomparator circuitry are not relevant to the instant invention, and theskilled person will be aware of and understand the various circuitconfigurations available for forming a comparator, no furtherdescription will be provided.

FIGS. 15 to 24 of the drawings illustrate oscilloscope plots of signalsobtained from the low pass filter 128 of the differential edge detectioncircuitry, i.e. signal 130 as illustrated in FIG. 7(d). However, due tothe electrical circuit configuration used in conducting the experiments,the oscilloscope traces illustrated in FIGS. 15 to 24 are inverted withrespect to the signal 130 illustrated in FIG. 7(d).

The results are obtained using edge detection circuitry comprising biascurrent measurement unit, high and low pass filters and comparatorssubstantially as described with reference to FIGS. 12, 13 and 14.

The results are achieved using a Planmeca dental X-ray source having therecommended 2 mm Al filtration and 30 cm focal spot to imaging device 16distance. The X-ray source was set to an 8 milliamp current, and 63 kVvoltage and of 10 millisecond duration. Measurements were made with200/2000 Hertz and 100/1000 Hertz high passflow pass filter settings.For each filter combination the comparator was kept constant at maximumsensitivity in order to inhibit any unwanted triggering. The imagingdevice received the clock signal, and the sensor and electronics wereoptically and electrically unshielded. The test results were achievedfirstly without an object, then with a dental phantom, then with 4 mmand subsequently with 12 mm Al targets.

For the plots illustrated in FIGS. 15 to 24, the imaging device was instandby mode and no readout or reset was performed. The signalcorresponding to pulse 178 of FIG. 7(d) is also marked 178 in FIGS. 15to 24. As expected, the amplitude of the pulses 178 is higher for the100/1000 Hertz configuration, compared to the 200/2000 Hertzconfiguration. This result should be expected from the function of ahigh pass filter. FIG. 15 has no object, and the oscilloscope traceclearly shows the rising and falling flank corresponding to the biascurrent. As the object absorbs and increases only the pulsecorresponding to the falling edge of the bias current exceeds the noiseflaw. This progression is shown in FIGS. 15 and 19 for no target and200/2000 Hertz and 100/1000 Hertz filter configurations respectively.FIGS. 16 and 20 for the dental phantom target indicate that there hasbeen a reduction in the flanks of the bias current, and FIGS. 17 and 21and FIGS. 18 and 22 respectively for a 4 mm and 12 mm thick Al targetshow continued gradual reduction in the bias current flanks as theabsorption level of the target increases.

Without any object, the triggering occurs before the falling edge. Thisis due to the sensitive adjustment of the comparator register anovershoot from the rising edge as well as a ripple of the X-rayintensity. Having an adequately selectable readout sequence shouldameliorate this problem.

FIGS. 23 and 25 show the oscilloscope trace obtained when the imagingdevice is in operation. Continuous readout/reset was performed on theimaging device. Due to relatively simple filtering the reset frequencyof the imaging device was about 30 kHz and became visible forcing ahigher comparator voltage, thereby reducing sensitivity. In the 200/2000Hertz configuration, the X-ray exposure cannot be detected with a 12 mmAl target but could be detected easily with a 4 mm Al target. In the100/1000 Hertz configuration, the X-ray exposure could still be detectedwith the 12 mm Al target, with respect to traces illustrated in FIGS. 23and 24.

In view of the foregoing description it will be evident to a personskilled in the art that various modifications may be made within thescope of the invention. For example, the system could be used fornon-destructive testing and analysis, as well as in medical imaging.Additionally, the high pass filter cut-off frequency may be determinedby the following relationship, f_(hpf)=2/t_(X-ray pulse duration), andlow pass cut-off frequency by f_(lpf)=2/t_(X-ray pulse rise/fall time).The bias measurement unit, high and low pass filters and comparator unitneed not be as specifically described above. In particular, all theelements after the bias measurement circuitry illustrated in respectiveFIGS. 6, 8 and 10 may be implemented in a Field Programmable gate Arrayor general purpose processor. However, for operation, e.g. data or clockrates greater than 100 kHz, an optimised Application Specific IntegratedCircuit should be used.

Although the X-ray source referred to in the foregoing descriptionprovides an initial X-ray pulse, the present invention is not limited touse with such X-ray sources, but to any form of X-ray source output andhigh energy radiation sources in general.

The scope of the present disclosure includes any novel feature orcombination of features disclosed therein either explicitly orimplicitly or any generalisation thereof irrespective of whether or notit relates to the claimed invention or emitigates any or all of theproblems addressed by the present invention. The application herebygives notice that new claims may be formulated to such features duringthe prosecution of this application or of any such further applicationderived therefrom. In particular, with reference to the appended claims,features from dependent claims may be combined with those of theindependent claims in any appropriate manner and not merely in thespecific combinations enumerated in the claims.

What is claimed is:
 1. A semiconductor radiation imaging assembly,comprising: a semiconductor imaging device including at least one highenergy direct conversion image element detector; said semiconductorimaging device comprising a semiconductor substrate supporting a firstand second conductive layer on respective first and second surfaces,said second conductive layer comprising an image element electrode andsaid first and second conductive layers at least partially opposing eachother for applying a bias therebetween to define a radiation detectionzone for said image element detector; and bias signal monitoring meansfor monitoring a bias signal applied to said first conductive layer fordetermining radiation incident on said image element detector.
 2. Animaging assembly according to claim 1, wherein said first conductivelayer comprises a substantially continuous layer across said firstsubstrate surface, and said second conductive layer comprises aplurality of image element electrodes for defining respective radiationdetection zones for a plurality of image element detectors.
 3. Animaging assembly according to claim 1 wherein said bias signalmonitoring means is adapted to provide a trigger signal for said biassignal fulfilling a predetermined criterion.
 4. An imaging assemblyaccording to claim 3, wherein said trigger signal is initiated inresponse to a transgression of a threshold value for said bias signalindicative of a start of a radiation exposure or end of a radiationexposure.
 5. An imaging assembly according to claim 4, wherein saidtrigger signal comprises a begin exposure trigger signal for saidthreshold value indicative of said start of radiation exposure.
 6. Animaging assembly according to claim 5, wherein said trigger signal isinitiated in response to said bias signal upwardly transgressing saidthreshold value.
 7. An imaging assembly according to claim 4, whereinsaid trigger signal comprises an exposure trigger for said thresholdvalue indicative of said end of radiation exposure.
 8. An imagingassembly according to claim 7, wherein said trigger signal is initiatedin response to said bias signal downwardly transgressing said thresholdvalue.
 9. An imaging assembly according to claim 1, wherein said biassignal monitoring means is adapted to determine a rate of change forsaid bias signal.
 10. An imaging assembly according to claim 9, whereinsaid bias signal monitoring means is adapted to discriminate betweenmore than one rate of change of said bias signal.
 11. An imagingassembly according to claim 1, said bias signal monitoring meanscomprising: a differentiator for differentiating a signal representativeof said bias signal; a low pass filter for low pass filtering saiddifferentiated signal; and a comparator for comparing said low passfiltered signal with a threshold value.
 12. An imaging assemblyaccording to claim 11, wherein said differentiator comprises a high passfilter.
 13. An imaging assembly according to claim 1, wherein said biassignal monitoring means is adapted to determine accumulated bias signalvalues representative of aggregate radiation incident on said imageelement detector.
 14. An imaging assembly according to claim 13, whereinsaid bias signal monitoring means is responsive to said accumulated biassignal value fulfilling a predetermined criterion to initiate a triggersignal.
 15. An imaging assembly according to claim 14, wherein saidpredetermined criterion comprises said accumulated bias signal valuetransgressing a first threshold value thereby providing a begin ofexposure trigger signal.
 16. An imaging assembly according to claim 14,wherein said predetermined criterion comprises said accumulated biassignal value transgressing a second threshold value to provide an end ofexposure trigger signal.
 17. An imaging assembly according to claim 16,wherein said bias signal monitoring means is adapted to subtract animage element quiescent bias signal value from a signal representativeof said bias signal.
 18. An imaging assembly according to claim 17,wherein said bias signal monitoring means further comprises sample andhold circuitry for recording an image element quiescent bias signalvalue; and subtraction means for subtracting said image elementquiescent bias signal value from said signal representative of said biassignal so as to form a quiescent bias signal corrected bias signal. 19.An imaging assembly according to claim 18, wherein said sample and holdcircuitry is resettable to update said recorded image element quiescentbias signal value prior to said bias signal monitoring means initiatingmeasurement of said accumulated bias signal.
 20. An imaging assemblyaccording to claim 16, said bias signal monitoring means comprising: anintegrator for integrating a signal representative of said bias signal;and a comparator for comparing said integrated signal with said firstand/or second threshold value.
 21. An imaging assembly according toclaim 1, wherein said bias signal monitoring means is adapted tointegrate a signal representative of said bias signal and to subtractsaid integrated signal from said signal representative of said biassignal so as to derive a signal representative of radiation incident onsaid image element detector.
 22. An imaging assembly according to claim21, wherein said bias signal monitoring means is adapted to integratesaid signal representative of radiation so as to generate an integratedsignal representative of radiation.
 23. An imaging assembly according toclaim 22, wherein said bias signal monitoring means is responsive tosaid integrated signal representative of radiation fulfilling apredetermined criterion to provide a trigger signal.
 24. An imagingassembly according to claim 23, wherein said predetermined criterioncomprises said integrated signal representative of radiationtransgressing a first threshold value to provide a start of exposuretrigger signal.
 25. An imaging assembly according to claim 24, whereinsaid predetermined criterion comprises said integrated signalrepresentative of radiation transgressing a second threshold value toprovide an end of exposure trigger signal.
 26. An imaging assemblyaccording to claim 25, wherein said bias signal monitoring meanscomprises a comparator for comparing said integrated signalrepresentative of radiation with said first threshold value and/or saidsecond threshold value.
 27. An imaging assembly according to claim 1,wherein said bias signal monitoring means is adapted to monitor biassignal current.
 28. An imaging assembly according to claim 1, whereinsaid bias monitoring means is adapted to monitor bias signal voltage.29. An imaging assembly according to claim 1, wherein said image devicecomprises a plurality of detector elements, and said bias monitoringmeans is arranged to monitor said bias signal for at least some of saidimage elements.
 30. An imaging assembly according to claim 29, whereinsaid bias monitoring means is arranged to monitor said bias signal forall of said image elements.
 31. An imaging assembly according to claim1, wherein said bias signal monitoring means is integral with saidimaging device.
 32. A semiconductor radiation imaging system,comprising: a semiconductor imaging assembly according to claim 1;control electronics coupled to said imaging assembly for receivingsignals, including trigger signals, therefrom; signal storage means forstoring signals coupled from said control electronics; an imageprocessor for processing signals coupled from said control electronics;and a display unit for displaying images provided by said imageprocessor.
 33. An imaging system according to claim 32, wherein saidcontrol electronics are responsive to a trigger signal from said imagingassembly to initiate an image frame selection from said signals storedin said storage means.
 34. A method for providing a self-triggerablesemiconductor imaging assembly, the assembly comprising a semiconductorimaging assembly according to claim 1, said method comprising:monitoring a bias signal for applying a bias to said image elementdetector so as to monitor radiation incident on said image elementdetector; and initiating a trigger signal conditional on said biassignal fulfilling a predetermined condition.
 35. A method according toclaim 34, further comprising determining a change in said bias signalcorresponding to a change in radiation incident on said image elementdetector.
 36. A method according to claim 35, further comprisingdetermining a rate of change for said bias signal.
 37. A methodaccording to claim 36, further comprising discriminating between morethan one rate of change for said bias signal.
 38. A method according toclaim 37, in which said trigger signal comprises a start of exposuretrigger signal for said rate of change indicative of a start ofradiation exposure.
 39. A method according to claim 36, in which saidrate of change is indicative of a start of radiation exposure or end ofradiation exposure.
 40. A method according to claim 39, in which saidtrigger signal comprises an end of exposure trigger signal for said rateof change indicative of an end of radiation exposure.
 41. A methodaccording to claim 34, further comprising determining an accumulatedbias signal representative of aggregate radiation incident on said imageelement detector.
 42. A method according to claim 41, further comprisinginitiating a trigger signal for said accumulated bias signal fulfillinga predetermined condition.
 43. A method according to claim 42, in whichsaid predetermined condition comprises said accumulated bias signaltransgressing a threshold value.
 44. A method according to claim 43,further comprising initiating a start of exposure trigger signal forsaid accumulated bias signal transgressing a first threshold value. 45.A method according to claim 43, further comprising initiating anexposure trigger signal for said accumulated bias signal transgressing asecond threshold value.
 46. A method according to claim 41, furthercomprising: determining an image element detector quiescent bias signalvalue; subtracting said image element detector quiescent bias signalvalue from a signal representative of said bias signal; and accumulatingsaid bias signal after subtraction of said image element detectorquiescent bias signal value.
 47. A method according to claim 34, furthercomprising: low pass filtering a signal representative of said biassignal; subtracting said low pass filtered signal from said signalrepresentative of said bias signal for deriving a signal representativeof radiation incident on said image element detector; low pass filteringsaid signal representative of radiation; and initiating a trigger signalfor said low pass filtered signal representative of radiation fulfillinga predetermined condition.
 48. A method according to claim 47, furthercomprising initiating a start of exposure trigger signal for said lowpass filtered signal representative of radiation transgressing a firstthreshold value.
 49. A method according to claim 47, further comprisinginitiating an exposure trigger signal for said integrated signalrepresentative of radiation transgressing a second threshold value. 50.A method according to claim 34, wherein said step of monitoring saidbias signal comprises monitoring bias signal current.
 51. A methodaccording to claim 34, wherein said step of monitoring said bias signalcomprises monitoring bias signal voltage.
 52. A semiconductor radiationimaging assembly, comprising: a semiconductor imaging device includingat least one high energy direct conversion image element detector; saidsemiconductor imaging device comprising a semiconductor substratesupporting a first and second conductive layer on respective first andsecond surfaces, said second conductive layer comprising an imageelement electrode and said first and second conductive layers at leastpartially opposing each other for applying a bias therebetween to definea radiation detection zone for said image element detector; and biasmonitoring means for monitoring a bias signal applied to said conductivelayer, said bias monitoring means being for use in controlling readoutfrom said radiation detection zone so as to determine radiation incidenton said image element detector.